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CoursesArray PatternsTwo Pointers

Boss: Trapping Rain Water

The classic hard problem, tamed: water at each bar is min(maxLeft, maxRight) - height, and two pointers settle whichever side's bound is already decided.

BossComplete codeDebugCode orderMultiple choiceTrace8 exercises, 10 min
AlgoFox Pro

Array Patterns is part of AlgoFox Pro

You can read the whole course outline here. The exercises are in the app, along with your streak, your mistake review, and the progress that carries across your devices.

Pro starts with a 3 day free trial. The app store decides who is eligible for an introductory offer, and a lapsed subscription does not get another one.

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Already subscribed in the app? Sign in with the same account and Pro works here too. Quick Play and Battle are free on the web, and so is the first course in every subject.